a) Field of the Invention
The present invention relates to pattern defect inspection, and more particularly to inspection of a defect of a pattern formed on reticles, photomasks, or wafers, or of a database representing a pattern.
b) Description of the Related Art
Various patterns are formed on semiconductor wafers during manufacture of semiconductor devices. Generally, a reticle or mask having a pattern of an original layout is optically transferred in a reduced size or in the same 1:1 size to a resist layer coated on a semiconductor wafer, and developed to form a resist mask. By using this resist mask as a process mask, the underlying layer is processed.
A plurality of photolithography processes are generally carried out for the completion of a single semiconductor device. A mask is used at each of these photolithography processes. The number of masks used is called a mask layer number.
As semiconductor devices are becoming of high density and high integration, the amount of pattern data has increased and the inspection time of patterns has increased. Even fine defects may affect circuit performance because patterns are becoming very narrow. It is therefore desired to detect even a fine pattern defect without fail.
A fine pattern actually formed by using a designed pattern becomes different from the designed pattern because of interferences of light in lithography process or the like during manufacture. For example, although corners of a pattern have right angle apexes when designed, the actual corners formed are rounded. The design data does not therefore coincide with the actual pattern with respect to corners.
If an actually formed pattern is inspected at high sensitivity by using a designed pattern as reference at high sensitivity, all corners are judged as defects. However, these corners are not real defects but are within allowance. Such defects will be called quasi defects. Since quasi defects are permitted to exist, these defects are desired not to be detected by defect inspection. In order not to detect such quasi defects by inspection, a rounding process or the like becomes necessary for corners of the design data.
Various types of processes including the rounding process are therefore required in detecting detects by comparison between a formed pattern and data in a database, so that it takes a long time to carry out defect inspection.
In the manufacture of semiconductor integrated circuits, a number of semiconductor chips having same patterns are usually formed on a single semiconductor wafer. These semiconductor chips (dies) are sequentially exposed with a stepper by using the same reticle pattern so that each die has the same pattern. By comparing the dies having the same pattern, an abnormal pattern accidentally formed on a particular die can be detected reliably. This comparison inspection of dies can be performed at relatively high speed and can provide a high defect detection sensitivity.
A reticle to be used for the exposure of semiconductor chips has in some cases a plurality of same patterns, for example, 2.times.2 patterns. In this case, comparison inspection of a plurality of areas of the reticle having the same patterns can also be performed similar to the comparison inspection of dies with the same pattern.
If a plurality of reticles, even if each reticle has a single same pattern, are formed, the comparison inspection of reticles can be performed at high defect detection sensitivity. This inspection is generally called a plate-to-plate inspection.
Such a die-to-die comparison inspection or plate-to-plate comparison inspection will be called hereinafter a same pattern comparison inspection.
The same pattern comparison inspection can be performed at high speed and at high defect detection sensitivity. However, for example, if a designed pattern itself has a defect or if dust is attached to the reticle during exposure and the same pattern with the dust defect is exposed to a number of dies, such defects cannot be detected by the same pattern comparison inspection.
In such a case, a defect inspection process described above is necessary, which process detects a defect by comparison of an actual pattern and design data in a database. However, comparison between different kind of patterns, such as data in a database and a pattern on a reticle, may result in a quasi defect described above if it is performed at high defect detection sensitivity, and a long time is required for a defect inspection work.
Techniques of comparison inspection of a die and database has been proposed, for example, in JP-A-3-51747 in which three object optical systems are provided, comparison inspection of dies is performed based on image data taken by the two object optical systems, and the comparison inspection of a die and database is performed through comparison between image data taken by the other object optical system and the image data obtained from a database. Although the die-to-die comparison inspection and the die-to-database comparison inspection can be performed concurrently, the comparison inspection process takes a long time because the image data in the database is different in representation from the image data of the die.
With an increase in integration density of a semiconductor integrated circuit device, the patterns thereof become complicated. There may occur such mistakes in the stage of designing the patterns that patterns which should be formed are omitted, and that patterns which should not be formed are formed. In such cases, the database itself includes mistakes.
When masks or semiconductor wafers faithful to such a database are formed, however, those mistakes cannot be detected by usual defect or fault inspection. A long time inspection becomes necessary to detect and correct such mistakes. Design processes for avoiding such design mistakes have been proposed therefore.
For example, in designing contact holes for connecting a pair of wirings, it is proposed to detect contact portions which have widened width in a wiring pattern, and to generate a contact hole pattern automatically in each contact portion.
In the case of automatically generating patterns in this way, however, patterns may be forcedly generated at positions where the patterns should not be formed. For example, when one wiring pattern is connected at one end to an underlying layer and at the other end to an upper layer, in designing contact holes for the underlying layer, a contact hole may be formed at a position where a contact hole for the upper layer should be formed.
From the viewpoint of the operation of a semiconductor device, the important point is not necessarily an absolute precision of each pattern. Rather, alignment of patterns between a plurality of layers often becomes more important than the absolute precision. For example, more important is correct alignment between a contact hole and a higher level wiring pattern, or electrical isolation between different wiring patterns.
Conventionally, each pattern has been formed to have a precision as high as possible and then is inspected to thereby ensure alignment between different layers.
Also, the reliability of database has been improved by inspecting the database itself and correcting the detected defects.